In the metallurgical industry, pulverized coal is generally injected as combustible into blast furnaces. It is important, in order to ensure good functioning of the blast furnace, that the pulverized coal is of good quality, i.e. that the pulverized coal has the right consistence, size and humidity level. The pulverized coal is generally produced in a grinding and drying installation, wherein raw coal is ground in a pulverizer and dried to the right humidity level before the resulting pulverized coal is fed to a hopper for storage or direct use in a blast furnace. It is known to subject the freshly ground coal to a stream of hot gas so as to dry the pulverized coal. The pulverized coal can e.g. be entrained by the hot gas from the pulverizer to a filter, where the pulverized coal is then separated from the gas and fed to the hopper. Part of the gas is recirculated and heated before it is reintroduced into the pulverizer.
For the correct functioning of the grinding and drying installation, it is important to monitor the temperature of the gas at the exit of the pulverizer. If the temperature is too high, there is a risk that the filter, downstream of the pulverizer, is damaged by the hot gasses. If this occurs, the filter can no longer function properly and must be repaired or replaced, entraining unscheduled process stoppage and undesired maintenance costs.
Known grinding and drying installations are provided with an emergency cooling system associated with the pulverizer, wherein, if the temperature at the exit of the pulverizer exceeds a predetermined threshold, the emergency cooling system injects water into the pulverizer chamber, thereby cooling the gas. Such an emergency cooling system is generally also linked to emergency shut-off valves, e.g. one arranged at the gas inlet into the pulverizer and one at the gas outlet of the filter, so as to cut circulation of the gas through the installation, thereby effectively shutting down the grinding and drying installation.
A major problem with this solution is that due to the shutting down of the grinding and drying installation, the whole pulverized coal producing process is stopped for a certain period of time, resulting in loss of production. When the process is then started again, further problems occur. Indeed, during a startup phase of such a grinding and drying installation, gas is fed through the system before raw coal is introduced into the pulverizer. This allows the individual components to be heated to the desired working temperature. When the raw coal introduction is then started, a sudden drop in temperature at the exit of the pulverizer occurs due to the addition of cold and wet material. The gas is then further heated upstream of the pulverizer to compensate for this temperature drop. However, in such a grinding and drying installation, there is a relatively long transition time, i.e. the time it takes the exit temperature to reach the desired working temperature after the sudden temperature drop. During this transition time, wherein the temperature is too low, the pulverized coal is not dried sufficiently, such that the pulverized coal produced by the grinding and drying installation during the transition time has a humidity level too high to be used in blast furnace. Indeed, during the transition time the grinding and drying installation produces unusable coal slurry instead of valuable pulverized coal.